In integrated circuits, it is often necessary to provide registers capable of storing binary information in non-volatile way.
For example, in memory integrated circuits, redundancy circuits normally provide for allowing the functional replacement of memory locations that, for different reasons, are detected as defective, with redundancy locations. The redundancy circuits include registers, referred to as redundancy registers, capable of storing addresses corresponding to the defective memory locations, in operation, the addresses of the defective memory locations are compared to the current address so as to determine whether the requested access is to be diverted to a redundancy location. As known, the check of the existence of defective memory locations is performed in the integrated circuit testing phase, and the addresses corresponding to the defective locations have to be stored in non-volatile way, so that the indication of such addresses is not lost when the integrated circuit is not powered.
Non-volatile registers are also employed within control circuits for controlling configurable circuit structures, such as voltage or current dividers or arrays of switches, for storing the selected configuration of such circuit structures.
The non-volatility of the information in absence of power supply is achieved by providing an electrically programmable, non-volatile, storage element of the type used for realizing EPROMs or flash memories. Such an electrically programmable, non-volatile, storage element consists of a MOS transistor with a floating gate in which electrons can be injected for modifying the transistor threshold voltage. A programming circuit associated with the non-volatile storage element allows programming the desired information thereinto.
Moreover, by associating a latch circuit with the non-volatile storage element, and performing a single read operation of the non-volatile storage element, the information is loaded into the latch, and the loaded information is retained in the latch without any current consumption as long as the necessary voltage supply is ensured.
Recently, devices have been proposed that are capable of storing in a non-volatile way two bits of information in two separate locations of a MOS transistor. Instead of a floating gate, these devices have a layer of silicon nitride capable of trapping the electrons. As known, these devices have electric terminals that alternatively act as a drain or as a source, depending on the operation mode (read and erase or program).
It is probable that these non-volatile storage devices will be used in place of the traditional floating-gate MOS transistors to realize non-volatile latch circuits. This will probably occur not only for exploiting the capability of storing two bits instead of only one, but also for reasons of manufacturing process compatibility when the non-volatile latches have to be integrated in a circuit in which the realization of silicon-nitride non-volatile storage devices is contemplated.